The field of the invention is electron beam systems, in particular, systems for electron beam lithography.
In the field of particle beam systems, such as those used for lithography, it is necessary to measure the total current in the beam. High gain (efficient) electron detectors developed for microscopy applications are prone to local saturation (meaning that the relation of output signal to input current changes significantly) when directly impacted by the beam of a lithography system. The high gain possible with scintillator/photomultiplier tube, diode/pre-Amp and microchannel plate detectors is necessary to obtain optimal signal to noise ratios in lithography systems because the total beam current is relatively low at 0.1-1.0 xcexcA.
In situations where the beam is dispersed, these detectors have performed very well. All three of the detector types listed have been successfully employed as backscatter detectors. When the electron beam in a lithography tool is focused, or nearly focused however, the current density can be as high as 100 A/cm2. Even though the total current is low, the concentrated nature of the beam can lead to local saturation effects in the detector.
Once the detecting medium has saturated, additional beam current produces no more light (scintillator), electron-hole pairs (diode) or photoelectrons (microchannel plate) and additional current in the beam will not produce any increase in the current output from the detector. This reduces the useful operating range of the detector in critical applications like knife edge beam blur measurements, where the output current of the detector must be proportional to the input current in order to preserve data integrity. For this reason lower gain detectors (e.g. unity gain Faraday cups) are often used as transmission detectors on e-beam lithography tools.
The most common alternative solution is to use a low gain detector with supplemental amplification, but the signal to noise characteristics of such an arrangement are inferior to those obtained with a higher gain detector that requires less subsequent amplification. Others have also attacked this problem by locating the detector far from the target image plane. However, in order to achieve a reasonable degree of defocusing a few hundred millimeters of separation is required for a typical beam with a semi-angle on the order of 10 mRAD. This much separation is generally not available and would place unreasonable constraints on the system mechanical designers.
In summary, the art has sought a compact system for measuring low total beam current in beams having a high current density.
The subject invention relates to the use of a high gain transmission detector located behind a diffusing member that spreads the electron beam current over the surface of the detector. A feature of the invention is the use of a thin diffusing member that absorbs only a small fraction of the incident beam.
Another feature of the invention is the use of a small drift distance between the diffusion member and the detector, so that the detection system can fit in a confined space.
Another feature of the invention is that the diffusing member functions as a pellicle, protecting the detector from particulate contamination and deposition of cracked hydrocarbons from the ambient that would build up and require detector replacement.